The present invention relates to a method for operating a PEM (proton electrolyte membrane) or DMFC fuel cell (direct methanol fuel cell) in the minimal or part load range.
In fuel cell systems which have a large number of cells, problems occur with distributing the reaction media from cell to cell and within a cell in the minimal or part load range. On account of the smaller quantity of reaction media required in these load ranges, the active cell surface is under-supplied, resulting in cell failures. Moreover, there are moisture problems at the active cell surface since, on account of the low flow rate caused by the small quantity of reaction medium, it is impossible for sufficient water to be discharged from the fuel cell. To avoid these cell failures in the lower load ranges, the fuel cell systems are usually operated in higher load ranges and thereby with a greater quantity of reaction media than is necessary.
Furthermore, in the case of DMFC fuel cell systems, methanol breakthrough occurs in low load ranges. In the process, the fuel methanol diffuses through the active cell surface (membrane), with the result that the efficiency is reduced in this load operation.